Leaving aside that there's nothing the matter with the grammar that's been flagged in the attached screenshot, you can see that Check grammar is turned off. I've also turned off System Preferences > Keyboard > Text > Correct spelling automatically, just in case. Before I report it as a bug, (which is, I suppose, the thing to do), I wonder if Stack Exchange has any other solutions, please.
I'm running macOS 12.5.1 on a 2021 14" M1 MacBook Pro
Additional information: I discovered, half by accident, that if I ask Mail to check the grammar of a different e-mail then it flags additional 'problems'. If you turn off grammar checking it reverts to just flagging this particular one.